Investments - Schedule of Gross Unrealized Losses and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 579,353	$ 319,654
Less Than 12 Months, Gross Unrealized Losses	(11,700)	(2,172)
12 Months or More, Fair Value	208,613	251,634
12 Months or More, Gross Unrealized Losses	(9,141)	(5,582)
Total, Fair Value	787,966	571,288
Total, Gross Unrealized Losses	(20,841)	(7,754)
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	561,537	313,795
Less Than 12 Months, Gross Unrealized Losses	(10,889)	(2,107)
12 Months or More, Fair Value	202,928	245,969
12 Months or More, Gross Unrealized Losses	(8,600)	(5,021)
Total, Fair Value	764,465	559,764
Total, Gross Unrealized Losses	(19,489)	(7,128)
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	61,275	40,117
Less Than 12 Months, Gross Unrealized Losses	(1,325)	(318)
12 Months or More, Fair Value	10,454	10,662
12 Months or More, Gross Unrealized Losses	(457)	(285)
Total, Fair Value	71,729	50,779
Total, Gross Unrealized Losses	(1,782)	(603)
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	90,188	50,447
Less Than 12 Months, Gross Unrealized Losses	(1,743)	(261)
12 Months or More, Fair Value	79,455	84,193
12 Months or More, Gross Unrealized Losses	(3,947)	(2,180)
Total, Fair Value	169,643	134,640
Total, Gross Unrealized Losses	(5,690)	(2,441)
Corporate
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	260,523	113,464
Less Than 12 Months, Gross Unrealized Losses	(5,309)	(846)
12 Months or More, Fair Value	64,506	66,954
12 Months or More, Gross Unrealized Losses	(3,177)	(1,793)
Total, Fair Value	325,029	180,418
Total, Gross Unrealized Losses	(8,486)	(2,639)
Commercial mortgage and asset-backed
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	94,429	53,965
Less Than 12 Months, Gross Unrealized Losses	(1,578)	(244)
12 Months or More, Fair Value	22,658	25,299
12 Months or More, Gross Unrealized Losses	(732)	(454)
Total, Fair Value	117,087	79,264
Total, Gross Unrealized Losses	(2,310)	(698)
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	3,024
Less Than 12 Months, Gross Unrealized Losses	0	(1)
12 Months or More, Fair Value	7,775	32,154
12 Months or More, Gross Unrealized Losses	(29)	(100)
Total, Fair Value	7,775	35,178
Total, Gross Unrealized Losses	(29)	(101)
U.S. Treasury securities and obligations guaranteed by the U.S. government
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	53,118	50,760
Less Than 12 Months, Gross Unrealized Losses	(913)	(430)
12 Months or More, Fair Value	18,080	26,707
12 Months or More, Gross Unrealized Losses	(258)	(209)
Total, Fair Value	71,198	77,467
Total, Gross Unrealized Losses	(1,171)	(639)
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	2,004	2,018
Less Than 12 Months, Gross Unrealized Losses	(21)	(7)
12 Months or More, Fair Value	0	0
12 Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	2,004	2,018
Total, Gross Unrealized Losses	(21)	(7)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	17,816	5,859
Less Than 12 Months, Gross Unrealized Losses	(811)	(65)
12 Months or More, Fair Value	5,685	5,665
12 Months or More, Gross Unrealized Losses	(541)	(561)
Total, Fair Value	23,501	11,524
Total, Gross Unrealized Losses	$ (1,352)	$ (626)